resets the RFM22 radio every 8 cycles, in case it goes weird because of low temperature

Hardware

The hardware consists of a custom PCB, a uBlox6G GPS receiver, an RFM22B transmitter, all controlled by an ATMega328.

Power budget

To reduce the power consumption as much as possible, the board runs at a low voltage, 1.8V.
The AVR clock frequency needs to be limited to 4 MHz, as this is the fastest the AVR can run at this voltage.
See here for more information on running at low-voltage / low-frequency mode.

Current estimates:

Component

current

Remark

RFM22B

27 mA

27 mA @ +11 dBm transmit

uBlox 6G

12.2 mA

22 mW @ 1.8 V Power Save Mode (1 Hz)

AVR

1.7

active current, 4 MHz, 3V

Total

41 mA

current drawn from the 1.8V boosted voltage

Assuming 1.2V average battery voltage and adjusting for step-up ratio (1.8V / 1.2V) and voltage converter efficiency (60%), this means about 103 mA average current draw from the battery.
An Energizer 1.5V primary lithium battery has a capacity of about 3000 mAh, so it is expected to last approximately 29 hours (3000 mAh / 103 mA).

Weight budget

Most commonly used for pico balloons seems to be the Qualatex 36 inch foil balloon, see [1].
This balloon weighs about 37g and has a volume of about 100 liter.
See here for a graph of reachable height vs payload weight and maximum free lift to reduce the chance of bursting.

A possible alternative balloon is a Qualatex 36 inch latex balloon, weighs about 35 gram and a volume of about 400 liter (fully inflated).